Process for automatically controlled carburizing of the surface layer of steel articles



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United States Patent PROCESS FOR AUTGMATICALLY CONTRQLLED CARBUREZING OF THE SURFACE LAYER 6F STEEL ARTICLES Urs Wyss, Zurich, Switzerland, assignor to Maag Gear Wheel & Machine Company Limited, Zurich, Switzerland, a company of Switzerland Filed Sept. 24, 1963, Ser. No. 311,134 Claims priority, application Switzerland, June 17, 1960, 6,934/60 20 Claims. (Cl. 148-165) This invention relates to the carburizing of the surface layer of steel articles, and is a continuation-in-part of the invention claimed and described in my prior application Serial No. 116,781 filed June 13, 1961, now abandoned.

Many processes for carburizing articles in appropriate gas atmospheres, that is, gas carburizing, are known and have many variations in practice. The known processes can be broken down into the following three main groups, namely:

(a) The carburizing gases are produced in an appropriate gas generator, then either supplied to the carburizing furnace with the required composition or supplied to the furnace as a carrier gas having a very reduced carburizing action. In the latter case, gas which has a stronger carburizing action is supplied to the furnace additionally.

(b) The carburizing gases are produced in the carburizing furnace by cracking of continuously supplied appropriate and, normally preferably, liquid substances. This group of processes will be referred to hereinafter as drip feed processes, even in cases Where the liquids are gasified before they enter the furnace.

(c) carburizing using a solid and a gaseous phase,

that is, by means of the C-COCO equilibrium, a

supply of carbon, for example, in the form of coke or wood charcoal or the like, being present in the heat treatment furnace. This group of processes will hereinafter be completely neglected since it lies right outside the scope of the process according to the present invention.

Group (a) processes have the great disadvantage that the operation of a generator may be uneconomical. It is known to use for gas carburizing one gas, having the approximate composition /sCO|- /sH possibly diluted with N as a carrier gas, and to add a second gas or vapour, or a gas produced by cracking or pyrolysis, which has a very strongly carburizing nature, such as propane. If the carrier gas methods of this group of processes have to be used to treat large surfaces, an enormous quantity of carrier gas, and a correspondingly large generator capacity, must be provided. The only way of maintaining the basic composition of the gas substantially constant during the carburizing process is therefore to use large quantities of carrier gas. The advantage of this process is to yield the possibility of automatic control of the carbon potential by controlling the content in the gas of one of the constituents thereof.

Group (b) processes have the advantage of simple apparatus and simple working, but have been unable, using the substances disclosed for the purpose in literature of i 3,2l,2% Patented Aug. 17, 1955 ice that there was no chance of controlling the carbon po-' tential by controlling the contents of a single constituent of the gas.

Other disadvantages of the heretofore known drip feed procedures are soot and supercarburizing (forming of excess carbides) on the one hand or too low a carburizing effect on the other hand, if the surface area of workpieces varies greatly from charge to charge. However, there are economical advantages of the drip feed method.

As the demand for carbon by the steel is dependent on the surface area to be carburized and as this demand is much higher when the work first reaches carburizing temperature and becomes smaller with increasing carburizing time, it is necessary to adapt the feeding rate of the carburizing liquid accordingly. One possibility consists of saturating the surface first by feeding an excess of the carburizing mixture, with the danger of forming soot, followed by a diffusion cycle to eliminate excessive soot and supercarburization. If the surface area varies from charge to charge it is nearly impossible to get a distinct case carbon content at the surface of the Workpieces.

Sampling of carburizing gas from the furnace chamber can only be done if enough gas is developed by feeding the carburizing liquid. Feeding this liquid is of course also dependent on the carbon demand of the steel surface to be car'ourized. If the surface is small, only a small feeding rate of the carburizing mixture is necessary and hence a too small amount of gas is developed to allow extraction. In this case, by pumping sample gas from the furnace chamber, air would be sucked through the joints into the furnace, spoiling the atmosphere and rendering impossible an effective control of the carbon potential in this way. It isfurther a usual practice to set a high carbon potential (1.2 to 1.3%) during the first period and a lower one (0.8%) for the second period of the carburizing cycle (see the accompanying FIG. 5). For this reason the feeding rate of the liquid carburizing mixture has to be reduced to very low values or has to be shut off completely during the second period, with the effect that although the surface to be carburized is large, only a small quantity of gas is developed inside the carburizing furnace. This quantity is mostly much smaller than the necessary amount for sampling gas and to compensate leakage.

l have found that, provided that particular, that is critical, conditions of gas atmosphere or of the substances which produce the gas atmosphere, as well as critical conditions of introducing them into the furnace are observed, a control of the feeding rate according to the instantaneou carbon demand and consumption of the steel surface to be carburized can very readily be provided in the drip feed process by sampling gas from the curburizing furnace, checking the content of one component thereof, preferably H O or CO which are critical for the carbon potential, by means of a suitable and heretofore known instrument and to feed the carburizing medium automatically. The general practice of the generator carrier gas processes cannot be applied faithfully to the drip feed methods and essential alterations are necessary. It is further not possible to combine both procedures without additional and new consideration. The use of a compound forming a carrier or pressure producing gas with a weak carburizing effect and a separately metered strong carburizing compound would need an uneconomical high feeding rate of the carrier compound to maintain a constant gas composition. But a constant gas composition is a necessity. I aspired to use a carrier gas-forming liquid 3 carburizing liquid is very slow or interrupted. The feeding rate of the carburizing compound should be automatically adapted to the carbon demand of the workpieces, which is dependent on the surface area to be carburized, further .on the time and temperature.

According to the invention, a process for controlling automatically the carburizing of the surface layer of steel articles by gas carburizing in a gas atmosphere formed in a furnace chamber by thermal decomposition is characterised in that a combination of separately fed substances of which one yields a carrier gas, that is, a gas merely producing a positive pressure, and :another yields a carburizing gas, is used, the combination being such that during the cracking and carburizing reactions, the resulting two gases provide substantially the same and a substantially constant gas composition, and the supply of the gas-providing substances, preferably of the substance yielding the carburizing gas, is controlled by continuously determining the content of one constituent of the furnace gases. Such determinations may be preformed by instrumentation.

There is used as the carrier or pressure producing gas liquids convertible into the carrier gas by thermal dissociation in the furnace, and as carburizing gas, that is, a gas which causes a considerable carbur-izing effect of the articles in contrast to the carrier gas which has little, if any, carburizing action, a substance such as provides the same gas composition by the carburizing reaction as the carrier gas, so that throughout the carbu'rizing treatment, the atmosphere in the furnace is constant regardless of the proportion of carrier-gas-forming substance to carburizing substance. -It then becomes readily and reliably possible to provide an exact adjustment and control of the evolution of carbon at all times, for instance, by the supply of carburizing gas, or of the substances evolving the same, being controlled in accordance with the CO concentration or with the dewpoint of the waste gases.-

The substances which provide the carrier gas and the car-burizing gas are preferably introduced as two organic liquids, either together or separately, into the hot heat carburizing furnace at temperatures higher than 800, usually between 850 to 1000 (1., and they may be evaporated before entering the furnace. The substances are dissociated in the furnace and yield the required gas atmosphere.

Referring to .the accompanying drawings? FIG. 1 is a diagrammatic sectional elevation of an apparatus for carrying out the process according to the invention;

FIG. 2 is a graph showing the consumption of carbon or equivalent amounts of isopropanol or ethyl acetate needed to secure the indicated growth of the case depth, depending on time and surface area for temperatures of 850 (3., 900 C., and 950 FIG. 3 is a graph showing the feeding rate of carbon or the equivalent feeding rates of isopropanol or ethyl acetate connected with the necessary consumption shown in FIG. 2 to secure the indicated growth of the carburized case;

FIG. 4 is a graph showing the CO-contents of the furnace atmosphere relative to varying ratios of various liquids providing the said atmosphere;

FIG. 5 is a graph showing the carbon penetration in the carburized case after different carburizing times at 950 C. for a nickel-chrome-carburizing steel; and

FIG. 6 is a graph showing the relation between dew point (water content) of the furnace gas, carbon potential and furnace temperature in the process according to the present invention.

Referring to FIG. 1, a retort 3, disposed in a furnace comprising masonry 1 and heating elements 2, is closed in gas-tight manner at a part 5 by an insulated cover t. Disposed within -the retort is a tray 6 bearing workpieces 7 which are to be treated. An intense gas flow produced 4% by a fan 8 flows round all the parts of the workpieces '7. The gas is caused to flow in the direction indicated by arrows by the arrangement comprising the fan 8, a .top battle 9 and a lateral bafi'le It A pipe 11 extends through the top baffle 9 and the cover t and conveys gas, from inside the baffles 9, 10, which burns with a flame 12 to give a check on the positive pressure in the furnace. The positive pressure in the furnace is satisfactory, provided that the gas remains alight at the flame 12. The cover 4 is provided with at least one pipe 13 for the supply into the retort 3 of liquid substances for producing the gas atmosphere. Connected to the pipe 11 is another waste gas pipe 14 through which gas can be pumped from the furnace for sampling and for supply to an analyser J15.

If the CO content of the gas is to be controlled, the analyser can be any appropriate apparatus which will de termine the CO content of the sample gas sufficiently accurately. If the H 0 content of the gas is to be controlled, the gas analyser must take the form of an apparatus for determining the Water content, preferably for determining the dewpoint of the sample gas. The measurement results of the analyser 15 (sensitive element) are supplied to an automatic control device 16 of the kind conventional in the control and regulation art. Depending upon the gas atmosphere in the furnace and upon the programme set up for the carbon potential, that is, the re quired carbon consumption, the device 16 operates a valve 17 disposed in a pipe from a tank 18 for the supply of a liquid yielding a carburizing gas. A liquid which provides the carrier or positive pressure gas is disposed in a vessel 19 and flows through a valve 20 to the common supply pipe 13 where the two liquids are mixed with one another, although they can be supplied separately. The two tanks 18, 19 are so disposed that, when the valves 17 and 20 are open, the liquids can drip into the supply pipe 13 by their hydrostatic pressure and without or with the use of an injection pump. The liquids or the mixtures thereof evaporate and are cracked thermally between the baffles 9, ltl and the wall of the retort 3. The gas mixture is circulated by the fan 8, as indicated by the arrows, the gas stream passing uniformly over all workpieces 7. N0 soot is deposited on the workpieces 7.

As a rule, it is advantageous to maintain the supply of carrier gas liquid substantially constant throughout the carburizing treatment, but this does not mean that there may not be cases where it is advantageous to vary the supply of carrier gas liquid. For instance, it may be advantageous to reduce the supply of such liquid during some part of the carburization treatment below the quantity supplied at the start of such treatment. As a liquid providing a carrier gas there is suitable and organic compound which yields, in cracking, a carrier gas having neutral or only slightly carburizing properties. The main constituents should be CO+H with small quantities of CO and H 0 corresponding to the water gas and equilibrium; a compound in which proximately 1:1, such as methyl alcohol or formaldehyde, is suitable for this purpose. Alternatively, aliphatic hydrocarbons, or monovalent or polyvalent alcohols, aldehydes or ketones having from 1 to 5 carbon atoms can be used provided that they are injected in a mixture with such a quantity of water that the atomic-ratio of (3:0 in the mixture 'is substantially as hereinbefore specified. Cheap methanol has given satisfactory results in all circumstances. 7

Where surface areas which are very large in relation to the furnace chamber have to be carburized, there can be used as the liquid for forming the carrier gas, that is, producing the positive pressure, a compound or a mixture of compounds in which the C10 ratio is greater than unity, for instance, a mixture of 1 mol of isopropanol and 1 'mol of methanol. The quantities of such substances or the C:O ratio is apmixtures which sufiice to provide a pressure slightly above atmosphere are small enough for their carburizing effect to be only very slight on large workpiece areas.

To provide the carburizing gas, an organic compound is used of a kind which yields a strongly carburizing gas in cracking. In the cracking reaction, intermediate products may be formed, such as methane which, of course, is a very strong carburizing agent, but the automatic control of the CO or H O contents keeps the methane contents below 2%. Less suitable are substances, more particularly aromatic compounds which lead to extensive carbon precipitation in cracking, that is, deposits of carbon or tarry products. More particularly suitable are substances in which the C:O ratio is greater than 115:1, preferably between 221 and 3:1 or even greater than 3:1. There can be considered, more particularly, aliphatic-hydrocarbons having from 1 to 20 carbon atoms, preferably 1 to 4 carbon atoms, and their monovalent and polyvalent alcohols, aldehyde-s, ketones, ethers, esters and the like, mixed, it required, with appropriate quantities of water, and preferably isopropyl alcohol, acetone or methyl acetate and ethyl acetate.

As has been found, and as has been hereinbefore described, the gas composition must be maintained substantially constant throughout the carburizing treatment if Methanol cracks at furnace temperatures above 800 C. into a mixture consisting mainly of /3 CO and /3 of H according to the idealized reaction equation:

If carburizing liquids are added to this gas mixture according to the carbon demand of the surface, the CO content changes very differently for dilferent carburizers with increasing carburizer consumption. This is shown in FIG. 4 in which the CO-content is plotted against the ratio of carburizing agent to methanol; the balance to 100% consists mainly of hydrogen. Ethyl acetate, and a mixture consisting of acetone and methyl acetate in a molar patio of 1:1 which is not shown in FIG. 4, do not, when used in any proportion with methanol, produce a deviation from the original basic composition of the gas, since the products of the carburizing reactions conform exactly to the cracking products of methanol. Besides CO and hydrogen, small quantities of CO H 0 and CH corresponding to the equilibrium conditions, are present For a condition of a C-potent ia'l of 1% and a furnace tem perature of 900 C. typical gas compositions resulting from these reactions are shown in the following table which shows the composition of the furnace .gas resulting from cnacking and carburizing reactions.

the evolution of carbon in the furnace is to be controlled automatically by controlling a constituent of the waste gas. To this end, of the possible substances for forming the carburizing gas, one is chosen which, after yielding carbon to the surface of workpieces by the cracking and carburizing reactions, leads to the same composition of the furnace atmosphere as is provided by the substance producing the carrier gas, that is the positive pressure.

FIG. 2 shows the time dependence of the consumption of equivalent amounts of carbon, isopropanol or ethylacetate for surface areas of workpieces from 1 to 75 m. and for temperatures of 850 C., 900 C. and 950 C. each for a growth of the effective case depth of the carburized layer as indicated by the dashed lines De. The corresponding feeding rate of the carburizers necessary to maintain the indicated growing rate of the case is shown in FIG. 3. The FIGS. 2 and 3 are valid for all types of the usual carburizing steel on condition that carbon potentials of 1.05% at a temperature of 850 C., 1.15% at 900 C. and 1.30% at 950 C. are maintained. They were established by a series of experiments and calculations and show that the feeding rate should be continuously diminished during carburizing, being highest at the beginning and becoming smaller with increasing time and case depth. In considering these quantitative relations it is easy to recognise the great difficulty raised if a distinct case surface carbon content, e.g. 0.85 i005 has to be guaranteed, especially if the surface area varies from charge to charge. From FIGS. 2 and 3 it follows further, assuming a constant flow of the carrier liquid, that the ratio of the carburizing liquid to the carrier liquid varies considerably with diiferent surface areas, times and temperatures.

The necessary feeding rate for methanol to compensate sampling gas and leakage and to maintain the desired positive pressure inside the furnace corresponds to 300 to 600 g. (approx. /3 to 1 /3 pounds) per hour. The feeding rate for the carburizing liquid follows from FIG. 3, but is automatically adapted to the instantaneous demand through continuously controlling the dew point or the CO content of the furnace gas as described.

It can be seen that the reaction gas is exactly the same for methanol, ethyl acetate and a mixture of acetone and methyl acetate in a molar ratio of 1:1. In one particular form of the invention therefore, methanol, as a liquid providing a carrier gas, is associated with ethyl acetate providing the carburizing gas. The idealized reaction equation for the canburizing reaction of ethyl acetate, neglecting the small amounts of CO H 0 and CH is: CI-I COOC H =Q%C+2CO+4H C is transferred into the steel surface. The same ratio of CO to H is formed as is already shown by the cracking reaction of methanol.

From FIG. 4 it follows further, that a mixture of 10 pants of isopropanol and 3 parts of water as a ca'nburizing agent in combination with methanol as a carrier gas forming liquid causes only small deviations in gas composition when the ratio increases considerably. The same is valid for acetone. Hence the use of these liquids in combination with methanol is also a particular form of the invention.

In one known process, pure propane vapour is admixed to the gas evolved by cracking methanol, and the mixture is supplied to the furnace. FIG. -4 shows clearly why the difiiculties arise when endeavours are made to control such a system by controlling one particular gas constituent, for example, C0 The widely varying CO contents, and the concomitant variations in volume ratios of t-he'other gas constituents, introduce so many factors of uncentainty into the measurement that this process cannot be used in practice. Only if the consumption of carburizing agent is fairly small can isopr-opanol be used as carburizer as well. Consequently, these substances, because of their low costs, may be of use in cartying the process according to the invention. into practical effect.

If, for instance, a mixture consisting of 1 mol of ethyl alcohol and 1 mol of water, or 1 mol of isopropyl alcohol and 2 mols of water is used as the liquid to provide the carrier gas, a similar diagram can be prepared showing that the compounds and associations just mentioned are very satisfactory for yielding the gas for carburizing.

If the surface which it is required to carburize is very large in relation to furnace size, acetone makes an excellent carburizing agent, for instance, in combination with a carrier-gasiorming or positive-pressure-maintaining mixture of methanol and isopropanol in the molar ratio of 1:1; the supply of acetone is controlled automatically and sothere is no change in gas composition throughout the carburizing process. However, if acetone is used the constant CO contents are 25%, compared with a combination of methanol and ethyl acetate where the CO contents are 33%. If, therefore, a particular carbon level is to be maintained in either combination, the CO or H O contents of the furnace gas must be controlled appropriately. Where the surface of workpieces is large, after the furnace chamber has been scavenged with a gas (Vs of CO and /a of H produced by cracking methanol, a small constant quantity of ethyl acetate (instead of methanol) can be supplied continuously just to maintain a slight positive pressure to compensate leakage and sampling gas. An extra quantity of ethyl acetate is supplied automatically through the agency of apparatus con trolled in accordance with the CO or H O contents. This step is another method of providing a constant carburizing atmosphere.

Carburizing steels to which this process has been applied contain 0.06 to 0.27% carbon, 0.10 to 0.60% silicon, 0.3 to 0.9% manganese, to 2.5% chromium, 0 to 4.5% nickel, 0 to 0.8% molybdenum. There is no doubt,

according to the experience with these steels, that the process can be applied to any type of steel used for carburizing.

FIG. 5 shows typical carbon penetration curves after 1 /3, 4, 12, 25 and 36 hours carburizing time .at 950 C. for a steel containing 0.12% carbon, 0.31% silicon,

0.52% manganese, 0.67% chromium, 3.15% nickel and 0.05% molybdenum. The carbon potential set during the first 25 hours was 1.2 to 1.25% and after 28 hours of carburizing time it was reduced to 0.85% to decrease the surface carbon content to the desired amount. The effect is shown by the penetration curve for 36 hours in FIG. 5.

FIG. 6 represents the relation between dew point and carbon potential for different furnace temperatures. The dew point can be set with the control instrument No. 16 of FIG. 1, according to this graph. This graph is valid for methanol as carrier gas forming liquid associated with .carburizing liquids like ethyl acetate, acetone plus methyl acetate (molar ratio 1:1), acetone alone, isopropanol (10 parts) plus water (3 parts) or for relatively small surfaces also for isopropanol alone.

The process according to the invention has considerable advantages over the prior .art systems, namely, exact maintenance of the required carbon potential and of the required penetration depth, independence from the size of the surface area to :be carburised and therefore from the required amount of carburizing substance, no deposits of soot or other residues on the workpiece surfaces and therefore no cleaning, very reduced gas consumption, no complicated and expensive fittings for producing, conveying, measuring and controlling large volumes of gas, lowcost and readily available substances for the gas atmosphere, no extra apparatus for gas generation such as sep arate gas generators, the gas being generated merely by the liquids specified dropping into the furnace chamber, possibility of controlling either of the liquid supplies, and continuance of furnace charge Without disturbances in operation and losses even though the supply or dispensing of the controlled liquid or the heating fails temporarily because of an interruption in electricity supply.

What I claim and desire to secure by Letters Patent is:

1. Process for automatically controlled carburizing of the surface layer of steel articles in a furnace chamber by gas carburizing in a gas atmosphere formed in such furnace chamber by thermal cracking at usual car-burizing temperatures (850l000 C.) characterised in feeding 8 into such chamber a combination of organic substances, of which at carburizing temperature one yields a gas, producing a positive pressure, and another yields a carburizing gas, the combination of said substances being such that, after the cracking and the carburizing reac-' tions, the gases resulting therefrom and providing the gas atmosphere in the furnace chamber have substantially the same composition as each other and maintaining such composition substantially constant by controlling the supply of the carburizing gas-providing substance by continuously determining the contents of one constituent of the gas atmosphere formed in the furnace.

2. Process according to claim 1, characterised in that the supply of the carburizing gas-providing substances is controlled by controlling the water content, by determining the dewpoint, of the furnace atmosphere.

Process according to claim 11, characterised in that the supply of the carburizing gas-providing substances is controlled by controlling the CO content of the furnace atmosphere.

4. Process according to claim 1, characterised in that methanol is used as the substance providing the pressure producing gas, and the substance providing the carburizing gas is ethyl acetate.

5. Process according to claim 1, characterised in that methanol is used as the substance providing the pressure producing gas, and the substance providing the car'ourizing gas is a mixture of isopropanol with water.

6. Process according to claim 1, characterised in that methanol is used as the substance providing the pressure producing gas, and the substance providing the carburizing gas is acetone.

'7. Process according to claim 1, characterised in that methanol is used as the substance providing the pressure producing gas, and the substance providing the carburizing gas is pure isopropanol.

8. Process according to claim 1, characterised in that the substance providing the pressure producing gas takes the form of small quantities of a mixture of isopropanol and methanol, and the substance providing the carburizing gas is acetone.

9. lroccss according to claim 11, characterised in that, when the furnace chamber has been scavenged by gas produced by cracking of methanol, the substance providing the pressure producing gas takes the form of small and constantly supplied quantities of ethyl acetate, and an additional quantity of ethyl acetate is used as the substance providing the carburizing gas.

it. In the drip feed process for carburizing the surface layer of steel articles by gas carburizing in a gas atmosphere formed in a furnace chamber by thermal dissociation of hydrocarbon substances, the method of automatically effecting the desired degree of carburization which comprises feeding into a furnace chamber a combination of hydrocarbon substances, one or" which yields a gas producing a positive pressure in said chamber, and another of which yields a carburizing gas, the combination of said substances being such that, after the cracking and the carburization reactions resulting therefrom, the gases providing the gas atmosphere in the furnace chamher have substantially the same composition as each other and maintaining said gas mixture substantially constant by controlling the feeding rate of the carburizing gas producing substance to compensate for breakdown thereof during carburizing treatment of the articles, in consonance with continuously determining the content of one constituent of the furnace gases throughout the carburizing treatment.

11. The process according to claim 10, wherein the hydrocarbon substances before thermal dissociation are liquid, at normal temperatures outside the furnace chamber.

12. The process according to claim 10, wherein the feeding rate of the substance yielding the carburizing gas during the carburizing treatment is controlled by the content of a constituent of the waste gas leaving the furnace chamber.

13. The process according to claim 19, wherein control of the feeding rate of the substance yielding the carburizing gas during the carburizing treatment is controlled by the water content of the furnace atmosphere.

14. The process according to claim 10, wherein control of the feeding rate of the substance yielding the carburiz ing gas during the carburizing treatment is controlled by the CO content of the furnace atmosphere.

15. The process according to claim 10, wherein methanol is used as the substance yielding the pressure producing gas and ethyl acetate is the substance providing the carburizing gas.

16. The process according to claim 10, wherein methanol is used as the substance yielding the pressure producing gas and a mixture of isopropyl alcohol with water is used as the substance yielding the carburizing gas.

17. The process according to claim 10, wherein meth anol is used as the substance yielding the pressure producing gas and acetone is the substance yielding the carburizing gas.

18. The process according to claim 10, wherein methanol is used as the substance yielding the pressure producing gas and pure isopropyl alcohol is the substance yielding the carburizing gas.

19. The process according to claim 10, wherein the W substance yielding the pressure producing gas takes the form of small quantities of a mixture of isopropyl al cohol and methanol, and the substance yielding the carburizing gas is acetone.

24). The process according to claim 15, wherein when the furnace chamber has been scavenged by gas produced by dissociation of methanol, the substance yielding the pressure producing gas is a small constantly supplied quantity of ethyl acetate, and an additional quantity of ethyl acetate is used as the substance yielding the carburizing gas.

References Cited by the Examiner UNITED STATES PATENTS 2,161,162 6/39 Harsch 148-16.5 2,329,896 9/43 Harsch 148--16.5 2,541,857 2/51 Besselman et a1. 14816.5 2,673,821 3/54 Stutzman 148-16.5

FOREIGN PATENTS 629,879 9/49 Great Britain.

OTHER REFERENCES Metal Progress, July 1, 1954, pages 98-102.

DAVID L. RECK, Primary Examiner. 

1. PROCESS FOR AUTOMATICALLY CONTROLLED CARBURIZING OF THE SURFACE LAYER OF STEEL ARTICLES IN A FURNACE CHAMBER BY GAS CARBURIZING IN A GAS ATMOSPHERE FORMED IN SUCH FURNACE CHAMBER BY THERMAL CRACKING AT USUAL CARBURIZING TEMPERATURE (850-1000*C.) CHARACTERISED IN FEEDING INTO SUCH CHAMBER A COMBINATION OF ORGANIC SUBSTANCE, OF WHICH AT CARBURIZING TEMPERATURE ONE YIELDS A GAS, PRODUCING A POSITIVE PRESSURE, AND ANOTHER YIELDS A CARBURIZING GAS, THE COMBINATION OF SAID SUBSTANCE BEING SUCH THAT, AFTER THE CRACKING AND THE CARBURIZING REACTIONS, THE GASES RESULTING THEREFROM AND PROVIDING THE GAS ATMOSPHERE IN THE FURNANCE CHAMBER HAVE SUBSTANTIALLY THE SAME COMPOSITION AS EACH OTHER AND MAINTAINING SUCH 